xenapi: send identity headers from glance plugin

Send identity headers while uploading/downloading images from glance. Related
to:

1) Add identity headers while calling glanceclient from nova
Ife02059abbdce7920f1f408b71e9745d777fa770

2) Pass identity headers received in glanceclient to nova
Ifbef582aa4e64a2e7a46db43a9cc6cf8c3531dbd

3) Pass identity headers received by glance api to glance registry
Ie5f07ed6dfeaa8428de4f79c4d40d182328e6ab4

NOTE: since this is a change to a Dom0 plugin API, it requires the lock-step
upgrade of both the compute-manager and the dom0 plugin.

DocImpact

Change-Id: I6d5e3448d2c0acc392d18e0b88cec25cf313da5b
Merge "xenapi: send identity headers from glance plugin"